/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.flink.table.api.constraints;

import org.apache.flink.annotation.PublicEvolving;

/**
 * Integrity constraints, generally referred to simply as constraints, define the valid
 * states of SQL-data by constraining the values in the base tables.
 */
@PublicEvolving
public interface Constraint {
	String getName();

	/**
	 * Constraints can either be enforced or non-enforced. If a constraint is enforced it will
	 * be checked whenever any SQL statement is executed that results in data or schema changes.
	 * If the constraint is not enforced the owner of the data is responsible for ensuring data
	 * integrity. Flink will rely the information is valid and might use it for query
	 * optimisations.
	 */
	boolean isEnforced();

	/**
	 * Tells what kind of constraint it is, e.g. PRIMARY KEY, UNIQUE, ...
	 */
	ConstraintType getType();

	/**
	 * Prints the constraint in a readable way.
	 */
	String asSummaryString();

	/**
	 * Type of the constraint.
	 *
	 * <p>Unique constraints:
	 * <ul>
	 *     <li>UNIQUE - is satisfied if and only if there do not exist two rows that have same
	 *     non-null values in the unique columns </li>
	 *     <li>PRIMARY KEY - additionally to UNIQUE constraint, it requires none of the values in
	 *     specified columns be a null value. Moreover there can be only a single PRIMARY KEY
	 *     defined for a Table.</li>
	 * </ul>
	 */
	enum ConstraintType {
		PRIMARY_KEY,
		UNIQUE_KEY
	}
}
